Ditto the possible problems.
Tumor or other growth has grown too big to keep the fish healthy, may be causing constipation or otherwise affecting the internal organs as large as it is. Euthanize the fish.
Organ failure for other reason: Even if you cured the reason (bacterial infection, virus, parasits) the organ damage has probably gone on too long to repair. Again, euthanize the fish.
Parasite: You could try curing this with anti-parasite medicines. Treat all the fish, they have all been exposed. Some chance of recovery.
However, once a fish reaches the point of not moving around much, lopsided swimming, and swollen area as shown there is not a good outlook.
Protect the other fish. Quarantine this one. Any disease or parasite may be leaving this fish and looking for a new host. Especially when the first host dies. Big release of infectious agents into the water.
Mycobacteriosis, either alone, or as part of a complex issue.
Isolate the fish, and if you can euthanize and check the fish yourself, do so, or take it alive to a fish specialist veterinarian to euthanize and stain properly to look for this disease. Treat the tank it came from as infected and use all possible precautions so you do not get infected with this.